(Not Applicable)
In a typical synthetic aperture radar (SAR), a series of coherent linear-FM chirped pulses is transmitted and received from a moving vehicle such as an aircraft or satellite. The received pulses are digitized and processed to form raw data that is Fourier Transformed to yield a complex image that is detected and displayed.
Radio frequency interference (RFI), which is a common name for electromagnetic interference (EMI), is any electromagnetic disturbance that interrupts, obstructs, or otherwise degrades or limits the effective performance of electronics/electrical equipment. It can be induced intentionally, as in some forms of electronic warfare, or unintentionally, as a result of spurious emissions and responses, intermodulation products, and the like.
SAR has progressed from its first demonstration in the early 1950""s to a precision instrument capable of generating 2 or 3-D radar images having a resolution in each direction of less than one foot. This high resolution requires a wide bandwidth signal, and a wide bandwidth means the SAR often receives RFI signals of a wide variety of frequencies and intensities. RFI is generated by garage door openers, cordless phones, two-way radios, pagers, wireless services, sensors, and many other sources. RFI routinely is detected by SAR as noise that masks the intended image.
The solution to this problem appears simple: remove the offending RFI. Most prior art techniques for accomplishing this solution generally fall into one of two categories.
One category includes attempts to filter the RFI from the raw data prior to image formation.
For example, H. Hellsten, U.S. Pat. No. 6,072,420, Jun. 6, 2000, discloses a radar system with a control unit which partitions the transmit-receive process into a number of consecutive sub-processes each of which consists of transmission followed by reception of a signal having a relative bandwidth of a fraction of an octave. The received signals from the different narrow band transmissions are used to reconstruct broad band radar data by pulse compression techniques. R. Goodman et al., U.S. Pat. No. 5,850,202, Dec. 15, 1998, discloses a SAR where a sequence of data processing operations initially uses a high number of bits to digitize radar echoes plus RFI, and then compresses the RFI tones, followed by additional processing which nulls the primary RFI contributors and finally requantizes the radar signal to a lower number of bits over the appropriate range of signal levels.
A variation of the first category taught by J. Garnaat et al., U.S. Pat. No. 5,546,085, Aug. 13, 1996, exploits any pulse-to-pulse coherence of interfering sources. A quadratic phase removal process removes quadratic phase variations contained in the interference to compress the interference to its narrowest extent in a range frequency dimension. While this technique works reasonably well for interfering broadcast television stations in the UHF band, many other emitters do not correlate well and, therefore, do not compress under this technique.
The second category attempts to model the RFI and cancel it in the raw data. For example, J. K. Jao, U.S. Pat. No. 6,166,678, Dec. 26, 2000, discloses a system where I/Q equalization is performed, which permits the data bandwidth to be reduced, thereby reducing RFI in the system.
One characteristic of all these systems is that they require modifications to the SAR hardware, although one skilled in the art can in fact often adapt many of these systems to software-only operations. More seriously, while these systems modify the raw data to remove RFI, they also are removing radar echo information and thereby degrading the SAR image, primarily by increasing offensive sidelobes in the range dimension.
H. Stankwitz et al., Nonlinear Apodization for Sidelobe Control in SAR Imagery, IEEE Transactions on Aerospace and Electronic Systems, Vol. 31, No. 1, January 1995, pp. 267-279, discusses a technique they call xe2x80x98dual apodizationxe2x80x99 where the complex image is processed at least twice using a different weighting functions each time. For each spatial location (or pixel), the minimum of the multiple values calculated from the multiple processes is utilized to form the image. If the proper weighting function is selected, the technique reduces sidelobes without reducing the mainlobe and is useful for sharpening a return from a bright target. Notably, this technique uses specific weighting functions for their particular sidelobe characteristics, presuming the entire spectrum exists. This technique does not filter RFI, nor does it accommodate effects of filtering arbitrary RFI, nor does it address utilizing RFI corrupted images to facilitate enhancing RFI-filtered images. A later paper by Stankwitz and Kosek, Sparse Aperture Fill for SAR Using Super-SVA, Proceedings of 1996 IEEE National Radar Conference, Ann Arbor, Mich., USA, May 13-16, 1996, proposes to fill in a spectrum gap (such as might be created by filtering RFI), but ignores entirely any notion of utilizing the original the RFI-corrupted data itself to facilitate sidelobe reduction.
Later work by H. Stankwitz et al. in U.S. Pat. No. 5,686,922, Nov. 11, 1997, utilizes dual apodization to improve the resolution of a SAR image. G. Thomas et al., SAR sidelobe apodization using parametric windows, SPIE, Vol. 3721, pp. 68-77, April 1999, discusses the effect of using different apodization filters.
It is an object of this invention to reduce interference without adversely affecting sidelobes using a characteristic of the interference rather than a preselected weighting function
It is a further object of this invention to use RFI filtering in an apodization-like system to improve a SAR image with minimal loss of information
To achieve the foregoing and other objects, and in accordance with the purpose of the present invention, as embodied and broadly described herein, this invention includes forming a first digital image from a phase-history data set that includes RFI and forming a second digital image after a frequency band containing the RFI has been removed from a phase history data set. The scaled images are then compared on a pixel by pixel basis; and a composite digital image is formed from the compared pixels having the lower magnitude.
Additional objects, advantages, and novel features of the invention will become apparent to those skilled in the art upon examination of the following description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.